How to extrude in the same way a "3D" angle?
What I do, and what Mr Zak mentioned in the comments, is to create a temporary face then extrude from that.

Simply select the end edge loop, and press F to make a face. 
Blender needs some direction to extrude along, making a face creates a face normal.

Now blender will automatically extrude along that face normal. Note that in order for this to work correctly the face must be flat.  

In this gif I first extrude with no face showing that there is no auto direction. Then I undo that, create a face and extrude again, this time along the new face normal.